i dunno how much help this will be at this point, but who knows, here goes:
What i had did a few days ago, I downloaded the "Red Dawn" campaign. I uploaded all of the .mis files from red dawn and put it into the missions/maps folder in GR:IT. I edited the campaign.xml file in the maps folder to accomadte the first RD mission to see if it would load. i also uploaded the art shell files into their respective directories. when i would go to campaign, it would load up the Watchful Yeoman mission, instead of the first RD mission, even though I changed the campaign.xml file to point to the first RD mission (i guess i have to change the cached file like Mike said). BUT, when i go to quick mission, all of the missions from RD show up, and about 3/4 of them have accompanying artwork. problem is, when i go to play the mission, i get an error saying to restart my xbox. what I assume the problem is, there were no mission briefings for any of the RD missions, and when you load up a mission, it goes to the mission briefings first, then to the mission. another thing I also tried and failed misserably at, I have the 4 new missions that came with the rainbow six 3 sampler disc installed. they are in the E partition i believe. each mission has its own folder, and so I created another folder and named it accordingly. i uploaded a mission to it, and changed some files so that it points to that mission. when i went to play the game, i got an error at the title screen saying "xxxxx06 is" shit, i forgot what it said, but it was an error basically saying that file was unusable and i can re-install that file (first extra mission) or i can delete that file. i just cut the xbox off, deleted that folder, and all is well again.